In a typical HgCdTe n/p photodiode, a heavily doped n+ type region is implanted in an upper surface of a vacancy doped p type HgCdTe semiconductor layer. Type conversion occurs by electrically active radiation damage, and by the liberation of mercury from the disrupted lattice and its subsequent diffusion to fill acceptor vacancies. Accordingly, any ionic specie may be used. The implanted n+ type region has a very high n type carrier concentration and extends only a few thousand Angstroms below the upper surface. Nevertheless, a diffused n type region extends much further into the HgCdTe semiconductor layer, resulting from the liberated and diffused mercury. In addition to implanting the n+ type region in this manner, an insulator layer is formed over the upper surface.
By design, the active junction is between the diffused n type region and the p type HgCdTe. However, the active junction can be very different near the upper surface. If the insulator layer has a high level of negative fixed charge, or if a negative guard ring bias is used, then the n type region near the upper surface becomes inverted to a p+ type region. The p+ type region abuts the implanted n+ type region, such that tunnel leakage current ("leakage current") is undesirably increased between the p+ type region and the implanted n+ type region. Such leakage current has been linked to 1/f noise. Accordingly, signal-to-noise performance of the photodiode is degraded, such that the photodiode is less effective at indicating an amount of radiation incident on the photodiode. In many cases, the active junction moves from the mildly graded n/p junction to the sharply graded n+/p+ junction. This phenomenon is referred to as "pinch-off".
A first previous technique attempts to reduce such leakage current by precisely establishing the fixed charge of the insulator layer. Under this first approach, the passivation requirements of nearly zero fixed charge and bake stability are very stringent. As a practical matter, it is very difficult to precisely establish the fixed charge of the insulator, and a deviation in fixed positive or negative charge can increase leakage current.
A second previous technique attempts to reduce leakage current by using a metal guard ring to set the potential at the upper surface. As a practical matter, it is very difficult to precisely set the surface potential of such a metal guard ring, and noise variations in the surface potential can increase leakage current. Undesirably, fabrication complexity increases and yield decreases, due to the additional required step of carefully forming the metal guard ring which is subject to being inadvertently shorted to other elements of the integrated circuit.
Thus, a need has arisen for a method of reducing leakage current in an integrated circuit, in which leakage current is reduced more effectively than in previous techniques. A need has also arisen for a method of reducing leakage current in an integrated circuit, in which passivation requirements are less stringent than in previous techniques. Further, a need has arisen for a method of reducing leakage current in an integrated circuit, in which fabrication is less complex than in previous techniques. Moreover, a need has arisen for a method of reducing leakage current in an integrated circuit, in which yield is higher than in previous techniques.